USPS Flat Rate Box Sizes – Dimensions, Costs, When to Use

USPS Flat Rate boxes ship anything up to 70 lbs anywhere in the US for one fixed price. No zone math. No dimensional weight calculation. No fuel surcharges. No nonstandard fees. The price depends only on which box you use.

That simplicity is the appeal. It is also the trap. Flat Rate boxes are free and the pricing feels predictable which leads sellers to use them on shipments where Ground Advantage or USPS Cubic would cost $5-15 less per package.

This guide covers every current box size, 2026 commercial and retail rates, the two critical fee exemptions most sellers do not know about, and the decision framework for when Flat Rate actually wins.

All USPS Flat Rate Box Sizes and 2026 Prices

All Flat Rate boxes ship via Priority Mail with 1-3 business day delivery, free tracking, and $100 insurance included. Boxes are free — you pay only the postage. Commercial rates are available through Pirate Ship, Shippo, ShipStation, and similar platforms at no extra cost.

Flat Rate Envelope
Dimensions: 12.5 × 9.5 inches (flexible thickness)
Retail: $11.95 | Commercial: ~$10.30
Best for: documents, flat accessories, items under 0.5 inches thick

Small Flat Rate Box
Dimensions: 8-11/16 × 5-7/16 × 1-3/4 inches
Retail: $12.65 | Commercial: ~$11.20
Best for: jewelry, small electronics, cosmetics, compact dense items

Medium Flat Rate Box — Top-Loading
Dimensions: 11-1/4 × 8-3/4 × 6 inches
Retail: $24.79 | Commercial: ~$19.60
Best for: bottles, candles, stacked books, upright items that load vertically

Medium Flat Rate Box — Side-Loading
Dimensions: 14-1/8 × 12 × 3-1/2 inches
Retail: $24.79 | Commercial: ~$19.60
Best for: clothing, framed photos, flat art prints, items that lay horizontally

Large Flat Rate Box
Dimensions: 12-1/4 × 12-1/4 × 6 inches
Retail: $34.02 | Commercial: ~$28.70
Best for: heavy tools, textbooks, multiple products, dense items over 10 lbs

All boxes support up to 70 lbs maximum weight. The price does not change between a 1 lb package and a 70 lb package in the same box.

Discontinued: Regional Rate boxes are no longer available. If you have old Regional Rate packaging, USPS will not honor Regional Rate pricing the shipment gets re-rated as standard weight-and-zone Priority Mail. Any guide still listing Regional Rate boxes is out of date.

The Two Medium Box Configurations – Same Price, Different Products

Both Medium Flat Rate Box configurations cost the same $19.60 commercial, $24.79 retail. But they are built for completely different products and the wrong choice wastes interior space.

Top-Loading Medium (11-1/4 × 8-3/4 × 6 inches):
This box is taller and deeper. It loads from the top. The 6-inch depth makes it ideal for products that stand upright: wine bottles, supplement bottles, candles, rolled posters in tubes, stacked books, or anything that fills vertical space naturally. If your product is 5-6 inches tall and relatively narrow, this is the right Medium.

Side-Loading Medium (14-1/8 × 12 × 3-1/2 inches):
This box is wide and flat. It loads from the side. The 14-inch width makes it the right choice for clothing, soft goods, framed photos, flat art prints, board games that are thin, or any product that lays horizontally. The extra 14 inches of width accommodates items that would never fit in the top-loading version.

The mistake: sellers default to the top-loading Medium because it is what their Post Office stocks. The side-loading Medium ships via mail order from USPS.com and is worth ordering if your products are flat. Packing a flat item into the top-loading box wastes the depth and often requires void fill that adds weight without adding value.

Two Fee Exemptions Most Sellers Do Not Know

Flat Rate packaging has two advantages that never appear in the rate comparison but add real value on specific shipments.

Exemption 1: DIM weight does not apply.
Standard Priority Mail and Ground Advantage apply DIM weight to packages over 1,728 cubic inches. Flat Rate packaging is completely exempt. A large lightweight item that would trigger DIM weight in your own box a bulky foam product, large pillow, light but oversized gift — ships at the flat rate regardless of its dimensional weight calculation.

UPS and FedEx have no equivalent exemption. A large lightweight package at UPS or FedEx gets billed at DIM weight regardless of how light it is. USPS Flat Rate eliminates that calculation entirely.

Exemption 2: Nonstandard surcharges do not apply.
USPS charges nonstandard surcharges of $4.50 to $21.00 for packages that are unusually long, large in volume, or awkwardly shaped. Flat Rate packaging is explicitly exempt from all nonstandard fees. A product that would trigger a $4.50 nonstandard surcharge in your own box ships at the flat rate with no surcharge.

Combined, these exemptions can add $5-20 of hidden savings on shipments that would trigger both DIM weight and nonstandard fees in standard packaging. Run the comparison before assuming Flat Rate is more expensive than your own box.

When Flat Rate Wins – The Zone and Weight Breakpoints

Flat Rate does not win on every shipment. It wins when a specific combination of weight, zone, and package density makes the fixed price cheaper than weight-based alternatives.

Flat Rate wins when:

Package is heavy relative to the box size. Flat Rate is priced by the box, not the weight. A 15 lb item in a Medium Flat Rate Box costs $19.60 commercial. The same 15 lb item shipped via Ground Advantage at Zone 5 costs approximately $25+. Flat Rate saves $5-6 per shipment.

Destination is Zone 6, 7, or 8. Flat Rate is zone-independent the same price coast to coast. Weight-based rates climb steeply at long distances. A 10 lb package at Zone 8 via USPS Ground Advantage costs approximately $25. The same package in a Large Flat Rate Box costs $28.70 only $3.70 more with 1-3 day delivery versus 2-5 day. At 8 lbs, the Large Flat Rate Box beats Ground Advantage at Zone 8.

Approximate weight breakpoints where Large Flat Rate Box beats Ground Advantage (Zone 5-8):

• Zone 5: break-even around 12-13 lbs
• Zone 6: break-even around 10-11 lbs
• Zone 7: break-even around 9-10 lbs
• Zone 8: break-even around 8-9 lbs

Below those weights, Ground Advantage is cheaper. Above them, Flat Rate wins.

When Flat Rate Loses – Three Situations to Avoid

Situation 1: Lightweight packages at short zones.
A 2 lb item in a Medium Flat Rate Box costs $19.60 commercial. The same item via Ground Advantage at Zone 3 costs approximately $8.26. Flat Rate costs $11.34 more on a lightweight short-distance shipment. This is the most common Flat Rate mistake using a free box because it is convenient, not because it is cheap.

Situation 2: Small dense packages that qualify for USPS Cubic.
If your product fits in a box smaller than 0.5 cubic feet with longest side 18 inches or under and weighs under 20 lbs, USPS Cubic pricing likely beats Flat Rate. A 10 lb item in a 10×8×6 inch box qualifies for Ground Advantage Cubic Tier 0.3 at approximately $9.20 at Zone 5 — versus $19.60 for a Medium Flat Rate Box. USPS Cubic saves $10.40 per shipment on that package. The USPS Cubic Calculator checks eligibility and shows the cubic rate alongside Flat Rate for direct comparison.

For a full breakdown of when USPS Cubic beats Flat Rate and Ground Advantage by zone and weight, see USPS Cubic vs Ground Advantage.

Situation 3: Products that do not fill the box.
Flat Rate charges by the box size regardless of how much of it you use. A product filling 30 percent of a Large Flat Rate Box pays the same as a product filling 100 percent. If your product is small enough to fit in a cheaper box, use the cheaper box. The free packaging is not an incentive to upsize.

How to Get Flat Rate Boxes and Commercial Rates

Flat Rate boxes are free from USPS. Two years to get them:

Online: Order from usps.com/store. USPS ships them to your door at no cost. The side-loading Medium box is typically only available online Post Office locations often stock only the top-loading Medium.

Post Office: Most locations stock envelopes, Small, top-loading Medium, and Large. Supply varies by location.

Commercial rates approximately 10-26 percent below retail are available through any USPS-approved shipping platform. Pirate Ship and Shippo offer commercial rates with no monthly fees and no volume minimums. ShipStation and Stamps.com also offer commercial access.

The retail-to-commercial gap on Medium Flat Rate runs approximately $5 per box $24.79 retail versus $19.60 commercial. At 100 shipments per month in Medium Flat Rate boxes, the retail premium costs $500 per month versus buying labels through a free commercial platform.

For the full comparison of Flat Rate versus Ground Advantage and USPS Cubic by zone, see Cheapest Way to Ship Small Packages 2026. For current Ground Advantage commercial rates by zone and weight, see USPS Ground Advantage Rates 2026.

Rates verified June 19, 2026. See changelog.

FAQ

Q: What are the USPS Flat Rate box sizes and prices in 2026?
A: USPS offers four main Flat Rate options. Flat Rate Envelope: $10.30 commercial. Small Flat Rate Box (8-11/16 × 5-7/16 × 1-3/4 inches): $11.20 commercial. Medium Flat Rate Box in two configurations top-loading (11-1/4 × 8-3/4 × 6 inches) and side-loading (14-1/8 × 12 × 3-1/2 inches) both $19.60 commercial. Large Flat Rate Box (12-1/4 × 12-1/4 × 6 inches): $28.70 commercial. All ship via Priority Mail in 1-3 days with free tracking and $100 insurance. Boxes are free from USPS.

Q: Is USPS Flat Rate exempt from dimensional weight pricing?
A: Yes. Flat Rate packaging is completely exempt from USPS dimensional weight pricing. DIM weight does not apply to Flat Rate shipments regardless of package size. Flat Rate is also exempt from USPS nonstandard surcharges of $4.50-$21.00 that apply to irregularly sized packages in standard packaging.

Q: When does USPS Flat Rate beat Ground Advantage?
A: Flat Rate beats Ground Advantage for heavy packages shipping to Zone 6, 7, or 8. For the Large Flat Rate Box at Zone 8, the break-even point is approximately 8-9 lbs above that weight, Flat Rate is cheaper than Ground Advantage. For Zone 5, break-even is around 12-13 lbs. For lightweight packages or short zones, Ground Advantage is typically cheaper.

Q: When does USPS Cubic beat USPS Flat Rate?
A: USPS Cubic beats Flat Rate for small dense packages that qualify for cubic pricing longest side 18 inches or under, 20 lbs or under, and 0.5 cubic feet or under. A 10 lb item qualifying for Ground Advantage Cubic Tier 0.3 costs approximately $9.20 at Zone 5 versus $19.60 for a Medium Flat Rate Box $10.40 savings per shipment. Use the USPS Cubic Calculator to check eligibility.